The present invention relates to transient voltage surge suppressors (TVSS) for protecting electrical equipment connected to alternating current (AC) electrical power lines, and in particular, to TVSS circuits for protecting electrical loads from sustained excessive AC voltages.
Electrical power lines are often subject to surges and other transients of high current or voltage caused by various events, such as lightning, switching on or off significant electrical loads, or even occasional short circuits. Such surges or transients can cause permanent damage to electrical equipment connected to the power line, particularly equipment in which electronic devices are responsible for the consumption and use of the power. Transient voltage surge suppressors have long been used to detect and attempt to divert such surges and transients before they reach the devices connected to the power line. The TVSS industry and marketplace generally focus on the limiting of transients of short duration, such as those resulting from lightning strikes. However, additional relatively common conditions exist which can produce sustained over-voltage conditions whereby the AC voltage presented via the power lines is significantly higher than that for which the connected electrical equipment is designed and capable of operating without damage. For example, poor voltage regulation by the electrical utility provider, improper wiring of the facility, use of standby generators, or incorrect or defective bonding of neutral and ground line connections can all lead to sustained AC over-voltage.
Under such sustained over-voltage conditions, conventional TVSS devices, which use voltage-limiting components (e.g., metal oxide varistors or “MOV”s) to limit transients of short duration, will either be ineffective (if their limiting, or “let-through,” voltage is more than the peak value of the power line voltage) or simply burn out, since MOV voltage limiters overheat in a very short time when absorbing the excessive power associated with the over-voltage.
It is therefore a first object of the present invention to provide an over-voltage protection circuit that disconnects a surge suppressor circuit and the load from the power source before the surge suppressor components are damaged, and then restores the load when the over-voltage conditions is abated.
Another object is to provide an under voltage protection circuit that disconnects a surge suppressor circuit and the load from the power source before the critical components in the device that constitutes the load are damaged, and then restores power to the load when the under-voltage conditions is abated.
Another object is to provide a variable voltage/time response, which varies in proportional to propensity for the condition to damage a MOV or other surge suppressor components, wherein the threshold for disconnecting the power from the surge suppressor circuit and load varies with the total energy in the voltage transient.
It is another objective to also disconnect the load and surge protector components under more sustained over voltage conditions, which would result in damage to the MOV, and/or require a thermal fuse coupled to the MOV.
Yet a further object is to provide a combined over and under voltage protection circuit, with the over voltage protection circuit having the aforementioned variable voltage/time response.
An additional object is to provide an over voltage protection circuit of the aforementioned character that precludes cycling of the disconnection circuit between the open and closed state at or near the trip threshold.